Wheelbase adjustment apparatuses and methods of the general type under consideration are known. Raising or lowering one or more vehicle axles changes the wheelbase of the vehicle and changes the loading of the other axles in a targeted fashion. As a result, it is possible, for example, to implement a starting aid by loading the wheels of the driven axles to a greater extent and, as a result, increasing the adhesion between these wheels and the underlying surface. Consequently, spinning of the driven wheels is made more difficult so that the starting process is made possible or made easier, particularly, on underlying surfaces that are covered with snow or ice. Furthermore, maneuvering the utility vehicle can also be made easier by raising one or more axles. The largest steering locks occur during maneuvering. As a result, large stresses occur between the axles, particularly, if two or more adjacent, non-steered axles are present. The reduction in the adhesion between the wheels of the raised axles and the underlying surface also reduces these stresses. Consequently, the wear on the wheels and the wheel bearings is decreased.
Conventional wheelbase adjustment apparatuses include one or more adjustment mechanisms, to which compressed air can be applied, arranged between the axles and the vehicle body of the utility vehicle. Applying compressed air to the adjustment mechanism(s) generates a pressure therein that correlates to the quantity of air that has flowed in. Conventionally, the quantity of compressed air that is applied to the adjustment mechanism(s) is controlled using, for example, a non-return overflow valve or a pressure limiting valve with afterfeed. These valves can be controlled manually by the driver or electronically. However, these valves tend not to maintain the set pressure value, particularly, if pressure peaks occur. Pressure peaks occur, for example, if an axle is to be lowered when the utility vehicle is under full load. If the pressure value is not maintained, this leads to a situation in which the desired relief of the loading on the other axles is no longer ensured, and the other axles are loaded to a greater degree than desired or even to a greater degree than is permitted. This causes increased wear on the wheel bearings on the other axles, and, in extreme cases, the bearings may fail. Furthermore, when these valves are used, a minimum pressure provided in the adjustment mechanism(s) may be undershot. The adjustment mechanism(s) at least partially perform the function of providing spring suspension to the utility vehicle. A minimum pressure is therefore necessary so that the adjustment mechanism(s) can generate their minimum necessary spring effect. If the minimum pressure is undershot, contact with the wheels without spring suspension—which contact occurs, for example, when traveling over potholes, railroad ties or bumps in the road—would be transmitted directly into the chassis arrangement which is consequently loaded to a high degree. As a result, the chassis arrangement wears more quickly or is damaged.
A method for maintaining minimum pressure in an air-spring bellows on a load bearing axle is described in DE 10 2004 036 251 A1. According to this reference, the stresses within the chassis arrangement are determined using a travel sensor, since the stresses bring about a change in the distance between the axles and the vehicle body. The adjustment mechanism(s) are actuated such that the stresses are reduced. The minimum pressure in the air-spring bellows is not determined directly. Instead, compressed air is always applied to the air-spring bellows such that a minimum distance between the axle and the vehicle body is not undershot. The problem of ensuring that the pressure value of the valves used is maintained is not solved by the method described in DE 10 2004 036 251 A1.